Conducting flight operations of any kind in airspace where aircraft not under one's direct control or where communications between such aircraft do not exist can be considered dangerous. Such situations may occur when unusual operations of aerostats (tethered balloons), unmanned aircraft, rockets or other aerial vehicles are flown in unrestricted airspace. For example, after a severe weather incident, flight of manned or unmanned aircraft is desired to examine the extent of possible damage to personnel or property on the ground. Because of the weather event, normal air traffic control services, which might have provided a risk mitigation strategy, are not available. Additionally, in the attempt to provide the greatest level of information dissemination, the area might not be restricted to media or other disaster relief aircraft. A system that would allow such activities to take place, while maintaining a high level of safety, would be invaluable to disaster relief agencies or others needing to conduct such operations.
In particular, the rapid development of Unmanned Aircraft Systems (UAS) in the United States and worldwide has generated investment in research, technologies, and systems at an unprecedented rate. The proven successes of Unmanned Aircraft (UA) in military environments and rapid advances in commercial navigation, automation, and sensing technologies have created new opportunities for civilian applications discussed in the preceding paragraph, of UAS. These opportunities have in turn imposed ever-increasing pressure on the Federal Aviation Administration (FAA) to respond with certification standards and regulations that will allow UA access to the National Airspace System (NAS) even in times of emergency. These standards and regulations, by necessity, will apply to all UAS operators.
Overshadowing all areas of this regulatory effort is the FAA's mandate to quantify and achieve a target level of safety that is consistent with the current level of safety of manned aircraft with respect to ground fatalities and mid-air collisions. As a risk mitigation strategy, it can be difficult to perform certain aviation operations and maintain that level of safety. “An equivalent level of safety” is a term heard often today. It implies that, in part, any system or operation that effectively replaces a particular capability of the pilots left on the ground, especially their ability to “see and avoid” other aircraft—must be as good or better than the pilot it replaces.
For Unmanned Aircraft (UA) operations (but it could apply to any operation that makes it more difficult to see and avoid other aircraft), FAA policy memo 08-01 gives Military, Public, and Private operators of Unmanned Aircraft Systems guidance on how operations are to take place and what approaches are allowed for Sense and Avoid (SAA). When unable to comply with Title 14 of the Code of Federal Regulations part 91.111 and 91.113, the operator may use alternative risk-mitigation strategies such as observers on the ground provided the UA is flown below three thousand feet above the ground (AGL) and within one mile laterally. If the UA is to be flown higher or farther from the observer, the UA must be chased by aircraft with an on-board observer. If the UA is flown in restricted airspace, however, no specific means of SAA is needed due to the nature of the airspace.
Although most UA will be equipped with GPS-based technologies, such as Automatic Dependent Surveillance-Broadcast (ADS-B) or other standard navigational aides, such technology cannot be assumed to be in place on other aircraft operating in the same airspace as the UA. This argues for the need for additional non-line-of-sight methods for monitoring the complete airspace desired for UAS deployment.
Operationally, radar systems have been used for the detection and deconfliction of aircraft for over fifty years. Recent advances in radar technology, along with corresponding advances in communications, computing, and data processing, now provide the capability for unprecedented real-time surveillance of large volumes of the atmosphere.